In a broadband communication system in recent years, a signal is used whose ratio (PAR: Peak-to-Average Power Ratio) between peak power and average power of a signal is about ten dB. In this case, in order to transmit information without an error, it is necessary to use the one which can transmit the peak power higher than the average transmission power by ten dB or more as a high frequency amplifier of a transmission stage. Generally, as for an amplifier, the larger the ratio between the peak power and average transmission power (back-off), the lower the power efficiency.
On the other hand, by the rise of environmental awareness, lower power consumption is also requested in a wireless system. Especially in a base station device of a mobile phone which is requested for high output of a few tens of W, a high frequency amplifier with a large power consumption ratio is desired to improve the efficiency. There are a Doherty type amplifier as a highly efficient high frequency amplifier and an amplifier of a drain modulation technique which synchronizes with an envelope of a signal and a drain voltage of an amplifying element.
In the Doherty type amplifier, only the carrier amplifier operates in a low output region, and both the carrier amplifier and the peak amplifier operate in a high output region. In the Doherty type amplifier, efficiency can be maximum in an output level in which the peak amplifier starts to operate, and thus it is possible to achieve high efficiency. Further, by changing a saturation output level ratio of the carrier amplifier and the peak amplifier, the output level in which the efficiency becomes maximum can be changed. However, in the low output region, the efficiency of the carrier amplifier biased to a class AB will be the efficiency of the Doherty type amplifier, and thus there has been a problem that the efficiency is reduced.
An envelope tracking amplifier which is an example of the drain modulation technique changes a power supply voltage of the amplifying element according to an envelope of a signal. For example, when the value of the envelope is small, lowering the power supply voltage maintains the low back-off state at any time, and thereby realizing a highly efficient operation. However, in a high frequency amplifier of a mobile phone base station, a signal band is several tens of MHz and a maximum output of the amplifying element will be several hundreds of W. Therefore, about 100 MHz high-speed operation, and several hundreds of W class of a high output operation are required, at the same time for a drain modulation power supply circuit which supplies electrical power to the amplifying element, and there has been a problem of increasing the circuit size.
PTL 1 describes a Doherty amplifier including a detector which detects an envelope of an input signal and a comparator which compares the envelope and a reference voltage. When the envelope is greater than or equal to the reference voltage, a pulse signal based on the size of instantaneous power of the envelope is output, and a drive signal according to the level of the output pulse signal is supplied to the carrier amplifier.